It has been decided, as part of the 3GPP standardisation process, that downlink operation for system bandwidths beyond 20 MHz will be based on the aggregation of a plurality of component carriers at different frequencies. Such carrier aggregation can be used to support operation in a system both with and without a contiguous spectrum (for example, a non-contiguous system may comprise component carriers at 800 MHz, 2 GHz, and 3.5 GHz). Whilst a legacy mobile device may only be able to communicate using a single, backward compatible, component carrier, a more advanced multi-carrier capable terminal would be able to simultaneously use the multiple component carriers.
Carrier aggregation can be particularly beneficial in a heterogeneous network (HetNet), even when the system bandwidth is contiguous, and does not exceed 20 MHz because multiple carriers enable interference management between different power class cells as well as open access and closed subscriber group (CSG) cells. Long-term resource partitioning can be carried out by exclusively dedicating carriers to a certain power class of cell (Macro/Pico/CSG).
Further, the need for interference management between different cells operating on component carriers of the same frequency in co-incident or overlapping geographic areas has led to the proposal (in Release 11) and development of extension carriers in which as much of the legacy control and pilot signalling (including Common Reference Signalling) as possible are removed. In addition to the benefits in terms of interference management, this also helps to minimise overhead. This means that extension carriers are not generally compatible with Release 10 and earlier mobile communication devices.
More specifically, a multi-carrier capable base station is able to operate at least one of its carriers as an extension carrier, on which a control channel (e.g. a channel carrying resource scheduling information such as the Physical Downlink Control Channel (PDCCH)), a Common Reference Signal (CRS) (sometimes referred to as a Cell-specific Reference Signal), and other information cannot be transmitted. To allow the use of an extension carrier, at least one further component carrier of the component carrier set used by the multi-carrier base station must be a stand-alone carrier that can be used to transmit the scheduling information for the extension carrier.
Accordingly, when a first base station is operating a component carrier as an extension carrier, another base station may operate a component carrier of the same frequency to transmit a control channel, a CRS and other such information more reliably, in the same general geographic area as the first base station, without significant interference because there is no corresponding control channel, CRS and other such information on the extension carrier operated by the first base station.
It is possible that the geographical areas covered by the primary cell and secondary cell may not be coincident—either by design, or because the range of the first and second component carriers is different as a result of radio environment conditions. In some cases the primary cell and secondary cells may be in a different geographical location altogether. Further, as mentioned above, the primary cell and secondary cells may operate in different frequency bands than one another. In such cases, despite the fact the primary and secondary cells are controlled from the same base station, a timing error and carrier frequency drift may, nevertheless, arise between the primary and the secondary cells.